Something labeled "virgin" you'd expect to be authentic beyond reproach—"extra-virgin" even more so. But in the case of olive oil, say experts, it's quite a slippery slope.

The evidence on olive oil has been piling up for several years. A 2011 study of extra virgin olive oil (EVOO) by the Olive Center at the University of California at Davis found that 73 percent of the five best-selling imported brands failed to meet the standards of taste and smell established for that grade of olive oil set by European regulators.

In a 2012 taste test, Consumer Reports found that only 9 out of 23 extra virgin olive oils tested met the tasting panel's standards. The two it rated "excellent" both came from the U.S.

Virgin olive oil, according to a staffer who worked on the report, is generally defined as being oil from olives not treated with chemicals, obtained from only the first pressing of the fruit (not by subsequent pressings of the left-over residue).

Extra virgin, he explains, is the highest grade of virgin, defined as coming from olives milled within 24 hours of harvest. It can range in color from yellow to green, must be free from defects and free from fatty acids. The oil must be obtained by physically milling or pressing the fruit--not by heating it. Additives are not permitted.

The same analyst explains that adulteration can take many forms: extra virgin olive oil can be cut with other seed oils, including soybean. Extra virgin can be blended with olive oils of lesser grades, on down to those "not fit for human consumption." Such grades of oil can be rendered fit for consumption by heating , which deodorizes and removes impurities. The resulting oil is relatively flavorless and on a par with vegetable oil.

Such adulterated products, according to the USITC, are widely being sold as pure, authentic extra virgin oil.

Kimberly Houlding, executive director of the American Olive Oil Producers Association, applauds the findings of the USITC report. "We believe consumers deserve to understand the quality of the oil they are buying and trust its authenticity," says Houlding. The fact that "extra virgin" does not, at present, guarantee oil of the highest quality, she says, works to the disadvantage of U.S. producers, who compete with cheaper imports on the basis of quality, more so than on price.

U.S. consumers, says Houlding, are being defrauded two ways: They're not getting what they think they are, in terms of quality. And they're not getting the heart-health benefits associated with extra virgin olive oil. "You won't get that from the lesser grades," she says.

"We've made a substantial investment in producing what we believe to be the best olive oil in the U.S.," says McEvoy general manager Dick Neilsen. "We find it frustrating to have to compete against other makers who represent their products as being extra virgin, when in fact their product is not comparable in taste, aroma or purity." He tells ABC News, "Nothing would please us more than to see agreement on uniform standards to ensure that when the bottle says extra virgin the consumer would know exactly what he or she was paying for."

Lax labeling standards, says the USITC staffer, make it difficult, if not impossible, for the consumer to tell one quality of extra virgin from another.

Stroll down the oil aisle of any well-stocked U.S. market, and you'll discover a belildering array of claims.

Your reporter, when he checked his own market this morning, found all the following extra virgin varieties, according to their labeling: cold pressed, cold extracted, hand-picked, first cold pressed, unfiltered, cold produced, packed in Italy, and "obtained exclusively from olives harvested in Italy." Prices ranged anywhere from 47 cents to 99 cents an ounce for domestic extra virgin, to 26 cents to 35 cents for imported.

The Trade Commission staffer says consumers should be wary of labels saying "packed in Italy," since olive oils from other countries, such as Tunisia, may be blended in. "I tell my friends, look for a single country of origin," he recommends.

When you come right down to it, however, there simply is no way—just by reading the label—to know the quality of what you're getting.

When might U.S. consumers be able to look forward to better labeling and quality standards? "Ask the guys on the hill," the staffer chortles, referring to Congress. The most recent farm bill contained, in draft, language calling for higher standards and better labeling. That language was later removed, reportedly at the behest of foreign producers and U.S. importers.